To centrally manage accounts you'd probably need NIS or LDAP (search for multiple posts on both). That would suggest a VPN from the remote sites to the central one. To run either over a WAN or worse - ADSL, would be asking for trouble.
IMHO a better solution would be to script an aggregation of accounts and then feed them out to all remote servers. In this case, ssh is your buddy.
You don't say why you want to allow users to log in to SME servers on different sites?
Damian